util: Fix assertion in iov_copy() upon zero 'bytes' and non-zero 'offset'

In cases where iov_copy() is passed with zero 'bytes' argument and a
non-zero 'offset' argument, nothing gets copied - as expected.

However no copy iterations are performed, so 'offset' is left
unaltered, leading to the final assert(offset == 0) to fail.

Instead, change the loop condition to continue as long as 'offset || bytes',
similar to other iov_* functions.

This ensures 'offset' gets zeroed (even if no actual copy is made),
unless it is beyond end of source iov - which is asserted.
Signed-off-by: NShmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Message-Id: <1470130880-1050-1-git-send-email-shmulik.ladkani@oracle.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

util/iov.c

@@ -247,7 +247,8 @@ unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
 {
     size_t len;
     unsigned int i, j;
-    for (i = 0, j = 0; i < iov_cnt && j < dst_iov_cnt && bytes; i++) {
+    for (i = 0, j = 0;
+         i < iov_cnt && j < dst_iov_cnt && (offset || bytes); i++) {
         if (offset >= iov[i].iov_len) {
             offset -= iov[i].iov_len;
             continue;